Laser employing elliptical reflector cavity



Dec. 14, 1965 3,223,944

LASER EMPLOYING ELLIPTICAL REFLECTOR CAVITY CL F. LUCK, JR-, ETALOriginal Filed Nov. 29. 1961 /V V E /V 70/?5 (24/?5/1 (5 f. A 06/6 weMAJ Wild ,e. 4 (45/1/0 M/A Afl (VF/34M A BY 71/ I I United States PatentCfiiice 3,223,944 Patented Dec. 14, 1965 3,223,944 LAlER EMPLOYlNGELLlPTICAL REFLECTGR CAVITY Clarence F. Luck, Jr., Waltham, Mass,Maxwell R. Krasno, Santa Barbara, Calif, and Mikael Ciftan, Wayland,Mass., assignors to Raytheon Company, Lexington, Mass, a corporation ofDelaware Continuation of application Ser. No. 155,651, Nov. 29, 1961.This application June 25, 1964, Ser. No. 383,538 1 Claim. (Cl. 33194.5)

This is a continuation of our abandoned application Serial No. 155,651,filed November 29, 1961.

This invention relates to devices for producing electromagneticradiation and has particular reference to improved means in opticalmasers for directing large quantitles of activating input radiation froma selected source onto a medium which is utilized to produce coherentstimulated output radiation.

In known types of optical masers wherein the input radiation is in theform of visible light, ultraviolet or infrared radiation, a reflectorcavity is provided for reflecting such input power from the sourcetoward the coherent radiation-producing medium. Known types of maserstructures embody a ruby rod, for example, as a source of coherentradiation which is located in the center of a helical flash tube used asthe input radiation source. In such structures, a considerable amount ofpower was required to operate the device, the input energy required perflash at room temperature being from 20003000 joules. Such energy rangesnecessitated the use of relatively large heavy power supplies.

We have found that extremely low efficiency results from loss of energyemanating from the flash tube due to the fact that most of the radiationfrom the flash tube is not intercepted by the ruby and, instead, isreabsorbed by the flash tube structure or simply radiated outwardly awayfrom the ruby.

Therefore, in accordance with this invention, We have provided a devicewherein substantially all of the radiation emanating from the flash tubesource is concentrated in the ruby. This is accomplished by providing asource of input radiation which is located in spaced relation with theoutput radiation member, both of which are enclosed within a cavity ofpredetermined design selected for reflecting nearly all of theillumination from the flash tube onto the output medium.

We have found that if instead of mounting the output source within anencircling input source, the two sources are spaced apart within areflecting cavity, great improvements are achieved in the quantity ofillumination which actually impinges upon the output source. Forexample, if a cylindrical cavity contains the two sources in parallelspaced apart relation adjacent the axis of the cylinder, the greaterpart of the illumination from the input source will reach the outputradiation member.

Even greater improvement results when the cavity has an ellipticconfiguration and the two sources are located at the two foci of theellipse. A device of the most efficient construction embodies anelliptic cylinder having the optically active output radiation elementalong one focal line and the activating input radiation source along thesecond focal line. This elliptic cylindrical shape is utilized when theinput and output radiation sources are line sources. When such sourcesare point sources, the cavity should be an ellipsoid of revolution withthe two sources located at the first and second foci. In such ellipticcavities having the two sources located at the foci thereof, all theillumination from the activating source falls upon the output radiationmember except the very small portion which falls behind the lamp in thedirection away from the member. This small shadow" portion falls backupon the lamp which comprises the activating source and is reabsorbed tosome degree by the lamp.

Accordingly, a primary object of this invention is to provide animproved device for producing electromagnetic radiation.

Another primary object is to provide a device of the above characterwhich utilizes substantially all of the illumination from an inputradiation source.

A further object is to provide a device of the above character which isself-contained, light weight, and relatively simple in design andoperation.

Other objects and advantages of this invention will become apparent fromthe following description taken in connection with the accompanyingdrawings, wherein:

FIG. 1 is a fragmentary elevational view partly in axial section showingone embodiment of the invention;

FIG. 2 is a sectional view taken substantially on line 2-2 of FIG. 1;and

FIGS. 3 and 4 are diagrammatic views of modified devices embodying theinvention.

A device for producing electromagnetic radiation requires that if thatoutput frequency is in the visible region, such as is the case in rubyoptical masers, the input power must be used very efficiently because ofthe limitation of available sources of input power. This invention isconcerned with a structure which is highly efficient when infrared,visible or ultraviolet input power sources or combinations thereof areused.

Optical masers require input power in the form of electromagnetic waveenergy or radiation whose frequency may range from far ultraviolet tothe far infrared. Furthermore, all masers have a resonant structurewithin those boundaries coherent stimulated radiation is produced. Themedium which produces this coherent radiation can be solid, liquid, orgaseous, and may have a variety of configurations. Furthermore,frequency of the output energy or radiation can be anywhere from theultraviolet to and including the microwave region.

No matter what other conditions exists, we have found that if the inputradiation is in the form of visible light, ultraviolet or infraredradiation or combinations thereof, best results are obtained when usinga reflector cavity which concentrates all of the input power into themedium which is used to produce the output radiation. Accordingly, for apoint-like input radiation source and a point-like medium which producesoutput radiation, we make use of an ellipsoid of revolution with thesource and medium in the first and second foci respectively. For a linesource, however, an elliptic cylinder is used with a line-type medium ofoptically active material along one focal line and the input radiationsource along the other.

Although not quite as efficient as a true elliptic reflector system,extremely good results are obtained by providing a cylindrical cavity10, as shown in FIGS. 1 and 2, having a central longitudinal axis 11. Aline-like source 12 of input radiation and a line-like output radiationmember 13 are located diametrically opposite one another on oppositesides of the axis 11 and are disposed substantially parallel with eachother and with axis 11. This structure places the output member outsidethe structure of the input source, as opposed to known prior artstructures wherein the input source is a helix having the output memberlocated coaxially within the turns thereof.

In the presently described structure the output member and input sourceare located as close to the axis 11 as is feasible in an attempt toapproach the optimum condition wherein both would lie on the axiswhereby all input radiation will be reflected back to the axis by thereflecting inner surface of the cylindrical cavity 10. Since it isimpossible to align both the input source and output radiation member onthe axis, closely spaced relationship with the axis 11 is provided.

The cavity may be any suitable material formed into a hollow cylindricalshape and having disc-like end plates 14 and substantially closing theinterior compartment formed thereby. The inner surfaces of the side andend walls are preferably all made to be highly reflective such as bypolishing or by deposition of mirror-like finishes thereon, or both.

The input source 12 is indicated as a flash tube which may be ofconventional or nonconventional design and is mounted in the structurein any suitable manner whereby substantially parallel relationship withaxis 11 and output member 13 are maintained. The ends of the flash tube12 are suitably connected into circuitry (not shown) whereby the tubemay be flashed to emit a bright burst of illumination for a selectedrelatively short period of time in the known manner of flash tubes ofthis type. The helical wire 16, wound in widely spaced coils about theflash tube 12, comprises a trigger for initiating the firing of thetube.

The output radiation member 13 is shown as a ruby rod but may compriseany other suitable optically active material capable of producing thedesired radiation, coherent being desired in optical masers of thecharacter described, and of being physically shaped into rod-likeconfigurations. Rod 13 is mounted in any suitable manner substantiallyparallel with axis 11 and with the axis of tube 12, and is shown withits ends supported by end walls 14 and 15. However, the end which is inend wall 14 is exposed so that illumination therefrom may be directedexteriorly of the device. An end cap structure 17 carrying a conicalport 18 may or may not be used, as desired. The ends of rod 13 may besuitably coated to increase light transmission from the rod in thedesired manner.

In accordance with this invention, when the flash tube 12 is operated,the resultant radiation, termed herein as input illumination, is emittedin all directions. It is obvious that some of this input illuminationfalls directly upon the rod 13. However, all of the illumination whichdoes not pass directly to the rod is directed upon the reflectingsurfaces of the cavity and is reflected back into the general area ofthe axis 11 where it falls upon the tube 12 or rod 13 or passes throughto an opposite wall where it is again reflected back in the generaldirection of the central area of the device. It is to be noted that theend walls, being highly reflective, aid considerably in reflectingillumination from the source toward the rod.

We have found that the energy required to operate this device isextremely low, in the order of less than 100 joules at room temperature,as opposed to about 2000- 3000 joules which is required to operate priorart types of optical masers. This permits the use of small powersupplies.

The considerable improvement which results from the presently describedstructure is attributed to the fact that most of the input illuminationemanating from the flash tube is concentrated in the ruby rod, whereasin prior art devices most of the input illumination is not interceptedby the ruby but is either radiated away or reabsorbed.

FIG. 3 diagrammatically illustrates a further improvement whereinreflecting cavity 19 has an elliptic shape, specifically a cylindricalellipse, and contains an input source 20 of activating radiation and anoutput radiation member 21 both of which are line-like (as opposed topoint-like) and are located on the two focal lines 20a and 21arespectively of the ellipse formed by the inner surfaces of the cavity.Here again the input source 20 may be a flash tube and the output member21 a ruby rod or the like. All of the radiation or illuminationoriginating from source 20 is focused onto the member 21, as shown byray lines and arrows, except for the small portion 22 of illuminationwhich falls behind the source 28 away from member 21. However, thisportion 22 which is lost is a very small fraction of the total radiationfrom source 20 and is minimized by proper design of the relativedistances between the source and member and other pertinent parametersof the ellipse. This type of configuration is the most efficient in theconsideration of reducing the input threshold energy. Optimum efliciencywill be realized when the flash tube and red are of the same size andshape.

Another structure embodying elliptical reflector cavity configuration isshown diagrammatically in FIG. 4 wherein the cavity 23 comprises threequadrants of an annulus whose cross section is an ellipse. The inputenergy source 24 and output radiation member 25, which again may be anysuitable devices such as a flash tube and a ruby rod respectively, arelocated along the two circular foci of the elliptical doughnut 23. In astructure of this configuration, the principles and functions of thevarious elements are similar to those set forth in the description ofthe device of FIG. 3 whereby more efficient utilization is made of theradiation from the input source, with relatively little power beingrequired to operate the device.

A device embodying the present invention has other advantages such aspermitting simple and diversified methods for cooling the flash tubes orruby rod, or both, by permitting use of selective reflectingcharacteristics in the coatings on the walls of the cavity whereby onlydesired frequencies may be reflected while undesired frequencies may betransmitted externally if desired, by permitting introduction internallyof other elements such as filters to selectively control desiredradiations, or by permitting cryostated mounting of the coherentradiation producing member in order to take advantage of certaincrystals which may have desirable properties at lower temperatures.

From the foregoing it will be apparent that all of the objects andadvantages of this invention have been accomplished by the provision ofan optical maser device wherein extremely large amounts of radiation aremade to pass onto the radiation producing member. It will be apparent,however, that many changes and modifications may be made in thestructures shown and described without departing from the spirit of theinvention as expressed in the accompanying claim. Therefore, the mattershown and described should be considered as illustrative and not in alimiting sense.

What is claimed is:

An optical maser for producing coherent electromagnetic energy,comprising a hollow cavity having the shape of a portion of an annulusand having an elliptical crosssectional shape and further embodying twoknown spaced parallel focal lines of circular shape therewithin andextending longitudinally thereof, a line-like source of activatingradiation of a known frequency located within the cavity along one focalline, and a line-like medium which produces coherent-electromagneticenergy of a different frequency from said activating radiation whenirradiated by said activating radiation located within the cavity alongthe other focal line, the relative size and spacing of said source withrespect to said cavity being such that only a very small fraction of thetotal radiation from the source is intercepted by the source and allremaining radiation from the source is transmitted to said medium theinterior surfaces of the cavity being highly reflective and the interiorof the effective portion of the cavity being void ofradiation-intercepting means whereby substantially all the radiationfrom the source will pass directly and by reflection from the source tothe medium which produces coherent electromagnetic energy, and

means for directing outwardly of said cavity substantially only saidcoherent electromagnetic energy.

References Cited by the Examiner UNITED STATES PATENTS FOREIGN PATENTS4/ 1938 Great Britain. 5/ 1964 Great Britain. 1/1960 Russia.

6 OTHER REFERENCES Collins et al., Coherence, Narrowing, Directionalityand Relaxation Oscillations in the Light Emission From Ruby, PhysicalReview Letters, volume 5, No. 7, Oct. 1,1960, page 303.

Columbia Radiation Laboratory, Research Investigation Directed TowardExtending the Useful Range of the Electromagnetic Spectrum, EighthQuarterly Progress Report, December 15, 1959, pages 14 and 15.

Laser Technical Proposal, LTP-100, Raytheon Co., Waltham, Mass.

JEWELL H. PEDERSEN, Primary Examiner.

